Fuel cell systems are increasingly used as a power source in a wide variety of applications. Fuel cell propulsion systems have also been proposed for use in vehicles as a replacement for internal combustion engines. The fuel cells generate electricity that is used to charge batteries and/or to power an electric motor. A solid-polymer-electrolyte fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, a fuel, commonly hydrogen (H2), but also either methane (CH4) or methanol (CH3OH), is supplied to the anode and an oxidant, such as oxygen (O2) is supplied to the cathode. The source of the oxygen is commonly air.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane. The electrons flow through an electrical load (such as the batteries or the electric motor) that is connected across the membrane. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+), and electrons (e−) are taken up to form water (H2O).
Hydrogen storage systems have been developed to provide hydrogen to the fuel cell stack. The hydrogen is generally stored in a storage vessel in gas and liquid phases under pressure and at low temperature. In some instances, gaseous hydrogen must be recirculated back to the storage vessel. However, reintroduction of gaseous hydrogen into the storage vessel can significantly increase the system pressure of the storage vessel. If the system pressure is too high, a pressure release device is activated and gaseous hydrogen is vented to atmosphere.
One traditional hydrogen storage system includes a cryo-shut-off valve that enables withdrawal of hydrogen (i.e., liquid) from the storage vessel. The cryo-valve is disposed within a vacuum isolated housing and includes a return device. The return device enables gaseous hydrogen to flow back into the hydrogen storage tank. The return device is activated when a set pressure is reached. Cryo-valves that include the return device are complicated and expensive.
Another traditional hydrogen storage system includes a check valve that is external to the vacuum isolated housing. The check valve enables gaseous hydrogen to flow back into the gas phase hydrogen within the storage vessel. As discussed above, reintroducing gas phase hydrogen into the storage vessel significantly increases the system pressure.